The present invention relates to improved crosslinking components for paint binders which can be crosslinked through transesterification, and to the process of producing the crosslinking components.
European patent application No. EP 00 12 463 A 1 describes heat-hardenable binder compositions which crosslink on stoving in the presence of a transesterification catalyst through transesterification of the hydroxy groups of a resin component, which is free from acid groups and ethylenically unsaturated groups, with a polyester, which is free from acid groups and carrying more than one .beta.-hydroxy group. As long as the resin component is a cationic water-dilutable resin, the binder system may be applied by electrodeposition. The introduction of the .beta.-hydroxy ester group, particularly favorable for transesterification reactions, onto the resin component according to this European patent application, is effected through reaction of a polycarboxylic acid anhydride with glycols, glycol monoethers, polyols, and/or, preferably, with monoepoxides. The preferred polyesters carrying .beta.-hydroxy groups are those prepared from trimellitic acid anhydride and a glycidyl ester of a saturated aliphatic carboxylic acid with from 9-11 C-atoms, the carboxy groups thereof being linked to a tertiary carbon atom, known as "glycidylester C 10 E" in the literature. On crosslinking of the aforesaid component, the glycols linked as .beta.-hydroxy esters are set free at stoving temperatures of between 150.degree. and 200.degree. C., and are thus eliminated from the coating. In addition to the relatively high stoving temperatures necessary for reaction even when using transesterification catalysts, the relatively high quantity of decomposition products is a serious disadvantage of the aforesaid type of crosslinking component. Furthermore, high-quality paint raw materials are split off and have to be eliminated from the paint film. This is a drawback for economical as well as for ecological reasons.
AT-PS No. 372 099 therefore, proposes to esterify the carboxy groups of crosslinking components with low molecular weight alcohols. The carboxy groups stem from special dicarboxylic acids, particularly of malonic acid. These dicarboxylic acid esters may be chain ends of oligomeric or polymeric esters.
It has now been found that the hardener function of such esters may be substantially improved if the transesterifiable ester groups are linked to an oligomeric or polymeric structure which will not undergo saponification.
Accordingly, the present invention is directed to a process for producing crosslinking components suitable for water-dilutable paint binders which will crosslink through transesterification, characterized in that a compound of the structure EQU X--CH.sub.2 --COOR Formula (I)
in which
X is --COOR, --CN or --COCH.sub.3 and PA1 R is an alkyl group with from 1 to 8 C-atoms
is reacted in a KNOEVENAGEL-reaction with a carbonyl compound, water being split off, and the resulting .alpha.-disubstituted alkylidene compound thereafter polymerized to a polymeric compound having a molecular weight of between about 300 and 6000.
Specifically, the present invention is directed to crosslinking components suitable for water-dilutable binders which will crosslink through transesterification characterized in that a compound of Formula (I) above is reacted in a KNOEVENAGEL-reaction with a carbonyl compound to provide a polymer having the general formula ##STR1## wherein X and R stand for the same radicals set forth above, and R.sub.1 is a hydrogen atom or an alkyl radical. n is a factor which multiplies the unit to provide a molecular weight of between about 300 and 6000.
The invention is further concerned with the use of the crosslinking components prepared according to the present invention in crosslinking binder systems which are crosslinkable through transesterification, and particularly in binder systems which are used in cathodically depositable ED paints.
The following reaction mechanism is theorized for the process of the invention: ##STR2## X, R, R.sub.1, and n have the above-defined values.
The crosslinking components prepared according to the present invention have substantial advantages over the products known in the prior art. For example, oligomeric or polymeric components with higher functionality with respect to the transesterifiable --COOR groups can be produced according to the present invention than is possible with the polyesters produced according to AT-PS No. 372 099. The higher crosslinking density obtained thereby provides improved resistance characteristics of the paint films. "Monofunctional" monomeric esters can also be used according to the process of the invention, such as aceto-acetic acid esters or cyano-acetic acid esters, since no ester functional groups are consumed in providing the increase in molecular weight. Furthermore, the preparation of the component, i.e., the KNOEVENAGEL-reaction and the polymerization, can be carried out in one step, only water being the by-product of the reaction which may be easily eliminated using normal azeotropic means.
The starting materials are the diesters of malonic acid with alkanols having from 1 to 8 C-atoms, preferably 1 to 4 C-atoms, such as dimethylmalonate, diethylmalonate, and n- or isopropyl malonate. In the same way the alkyl or cycloalkyl esters of cyano-acetic acid may be used, such as the methyl-, ethyl-, propyl-, butyl-, 2-ethyl-hexyl-, cyclopentyl-, cyclo-hexyl cyano-acetic acid ester or methyl or ethyl-aceto acetic acid ester.
The preferred carbonyl compound is formaldehyde, particularly in its polymerized form, paraformaldehyde. The use of higher aldehydes, such as acetaldehyde or a butanal or an aromatic aldehyde is possible but provide no special advantage as compared to the inexpensive and readily available formaldehyde. Furthermore, ketones such as methylisobutyl ketone or cyclohexanone can be employed.
Advantageously, the crosslinking component is prepared by adding the paraformaldehyde in portions at about 60.degree. to 70.degree. C. to the ester and dissolving the paraformaldehyde therein. About 0.1 to 1 mole-% of a mixture of piperidine and formic acid are added as catalyst. The reaction temperature should not surpass 90.degree. C. until the paraformaldehyde has dissolved. Then the reaction temperature is slowly raised to 120.degree. to 150.degree. C., and the reaction water as it is formed is stripped from the reaction mixture through azeotropic distillation with an entraining agent, such as an aliphatic or aromatic hydrocarbon. The reaction is monitored with the aid of the reaction water which is forming. The simultaneous polymerization is monitored through a determination of the refractive index or the viscosity.
Optionally, the polymerization can be completed by adding a peroxide initiator. The crosslinking component, after elimination of the entraining agent, can be used substantially free from solvents. Optionally, it may be of advantage to use inert solvents for better handling or distribution in the basic resin. The products have a molecular weight of between about 300 and about 6000.
Suitable resins for use with the crosslinking component are resins which carry a sufficient number of groups capable of esterification or amidation, in order to guarantee a sufficient crosslinking of the paint film. Suitable resins of such type are disclosed in EP No. -00 12 463 or AT-PS No. 372 099. The preferred resins are the epoxy resins; and for cathodic deposition, the particularly preferred resins are the epoxy resin amine adducts which may optionally be modified to provide internal flexibility.
Processing of the binder systems to provide water-dilutable coating compositions, i.e., neutralization with acids, dilution to the concentration for application or, as in most cases, the co-employment of pigments and extenders, and the possible methods of application are known to those skilled in the art. A preferred method of application of the coating compositions containing the crosslinking components of the present invention is cathodic electrodeposition (CED), whereby the object to be coated is wired as the cathode of the system.
The following examples illustrate the invention without limiting its scope. Parts are by weight and, unless otherwise stated, refer to the resin solids content of the resin components.